Microwave oscillator apparatus comprising a velocity modulation tube



June 27, 1950 D. E. SUNSTEIN 2,512,980

' MICROWAVE OSCILLATOR APPARATUS COMPRISING A VELOCITY MODULATION TUBEFiled Aug. 31, 1944 WW Z l INVENTOR! Jay ZJa/ZJZZZM BY W6W Patented June27, 1950 MICROWAVE OSCI IQLATOR APPARATUS COMPRISING A VELOCITY MODULA-N TUBE David E. Sunstein, Elkins Bark, Pa.-, assignor to PhilcoCorporation, Philadelphia, Pa., a corporation of PennsylvaniaApplication August 31, 1944, Serial No. 552,143

4 Claims. (01. 315739) The present invention relates to microwavegenerators and more particularly to oscillators utilizing velocitymodulation tubes at harmonic frequencies.

At the higher frequencies in the microwave or ultra high frequency rangecertain factors tend toseriously limit the operation. At the higherfrequencies due to the small dimensions. of resonant cavities, it hasheretofore been substantially impossible to employ external mechanicaltuning. At the high frequencies the resonant cavity size becomes sosmall that the vacuum tubes employed are smallin dimension thus limitingthe power dissipation. In certain cases it also would be desirable toprovidemotor driven sweep oscillators operating at frequencies in excessof live thousand megacycles, but at those frequencies the fundamentalresonantcavities have heretofore been extremely small. I I In order toobviate the above mentioned disadvantages and other disadvantagesencountered in the generation of microwave frequencies in the higherfrequency ranges, thefcollector cavity of a velocity modulation tubeisarranged to be resonant at two frequencies harmonically related toeach other, and the cavity is so designed that the output energy can bedelivered only at the higher of the two frequencies. By such anarrangement a larger cavity size is possible, larger vacuum tubes may beemployed, greater, power dissipation is obtainable, and other operationssuch as tuning are greatly facilitated. A magnetic or electric tuningprobe may be used to vary the fundamental frequency and the harmonicfrequency generated. By utilizing a collector cavity which is resonantto two frequencies harmonically related to each other, the conventionalarrangement of a velocity modulation tube having buncher and catchercavities may be applied, or. thereflex type of tube having but a. singlecavity may be used. i

It, therefore, is an object of the present invention to provide animproved microwave generator arranged to operate at a harmonic frequencyand to deliver power only at the harmonic frequency. It is anotherobject of the present invention to provide a microwave generatorutilizing a velocity modulation tube which has a resonant cavityresonant attwo frequencies harmonically related to each other. r 1 a Itis still another object of, thepresent invens tionto provide an improvedmicrowave generator operating at the higher ranges of the ultra high,frequencies which is capable of supplyinggreater amounts of power. V I.t

2 Stillanother object of the present invention is to provide an improvedmicrowave generator operating at the higher frequencies which mayutilize mechanical tuning means. I

Other and further objects of the present invention subsequently will" bedescribed by reference to the followingdescription taken in connectionwiththe accompanying drawing wherein Figure 1 is a diagrammaticrepresentation of the. application of the present invention to'avelocity modulation tube utilizing two resonant cavities;

Y a Figure 2- is a cross sectional view of one manner of constructing acavity in accordance with the present invention;

' Figure 3 is a cross sectional view of the cavity as seen in thedirection of the arrows along the line 3 3-of Figure 2; and 1 Figure 4shows the application of the present invention to a velocity modulationtube operating in a reflex circuit;

Referring to Figure 1 of the drawing there is shown diagrammatically avelocity modulation vacuum tube ll having a glass'envelope l2. The glassenvelope 12 encloses a cathode l3 and a heater [4 therefor; focusingelectrodes 15, and two sets of grids l6 and Il extending to the exteriorof'the envelope for connection to suitable resonant'cavities. It isapparent that the grids may be omittedyif desired, and the cavitiescoupled to the electron stream inductively. However ordinarily the useof the grids is preferable. A- resonant cavity-l8 is connected to thefirst set of grids l6. The resonant cavity [8 is coupled by a-coaxialline ISto a resonant cavity 20 which is coupledthrough an iris Zl-to awave guide 22. Suitable potentials are supplied to the electrodes of thevacuum tube as is customary in oscillators utilizing two resonantcavities. The resonant cavity 20 is so constructed as to be resonant attwo frequencies harmonically related to each other, and the iris 2iinterconnecting the cavity 20 withithe wave guide 22' raises the Qof'the cavity at the harmonic frequency. The iris 2| serves asanimpedance matching element which insomecapplications maybe omitted. Itis possible to deliver power to the wave guide 22 under .such' operatingconditions because the bunched .electrons passing between the grids I!have a wave shape of electron density vs. time which. is rich .inharmonics under the proper direct current acceleration conditions.

Figures 2 and 3 show one manner. of constructing the resonant cavity 20of Figure 1. In these ,figures the resonant cavity 20 'has'a rectangularcross section. Opposite walls of the cavity 20 are provided withsuitable apertures 24 which may be provided with suitable contactmembers 25 for engaging the grid ll of the vacuum tube l l. Thefundamental frequency of the cavity 20 is determined primarily by thecross sectional dimensions A and B, by the spacing C between the gridsIT and the diameter of the grids D of the cavity if the dimensions L-Iand L-2 from the center of the aperture 24 to'the ends of the. cavity21!] are much greater than the dimensionw B. The dimension B is selectedso as to be small enough to prevent propagation of the funda mental modein the wave guide-.' The harmonic waves at the grids may be short'enoughin wave tive side of the anode supply of four hundred length topropagate in both directions until reaching the end walls 26-and'21.

cavity to couple witdsonie lines of'm'agneti'c.

force or -tnerundamentm mode to produce feed back of the fundamentalm'ode' in the cavity Ill of "Figure 1. The cavity l8of Figure 1 maybeof"conventional'design or 'ma'y be constructed similarly to cavity 20.

From" the representatien" of one manner of constructing the resonantcavity 20-itwill be readily appreciated that magnetic or electrictuningdevices may be used which'are 'particularly desirable inconnection with motor driven sweep oscillators operating at "frequenciesin excess' of five thousand megacycles' Since the 'cavity'zll may beconstructed with physical dimensions considerably larger than thedimensions obtained at the harmonic frequency, largervelocity'modulation tubes may be employed to obtain greater poweroutput. Vacuum tubes may be employed so that the fundamental frequencyof the cavity 2 i in the vicinity of the upper frequency limit of theparticular "tube and hence large output may be obtained at the harmonicfrequency because of the small power absorption, due only to cavitylosses, at the fundamental frequency.

The advantages obtained by the construction heretofore described andshown as appliedto the two cavity velocity modulation tube may also beobtained with a single cavity velocity'modulation tube arranged in theconventional reflex circuit. This is diagrammatically illustrated'in'Figure 4 wherein the velocity modulation tube '31 is of the'type havingaglas envelope 32 enclosing a cathode 33, a heater althere for focusingelectrodes 35, a repeller electrode 36' and a'pair of grids'37 adaptedto'be connected to a resonant cavity. The grids 31' may be" connected toa resonant cavitysuch as the'resonant cavity 2!! constructed inaccordance with the foregoing disclosure. The reflex circuit is onewhich applies th'eusual potentials to the vacuum tube 3| so as to obtainthe necessary feed back. It will be noted that theresonant cavity 20shown'in Figure 4is similar to that shown in Figure I withthe-"exception that the cavity is not coupled to another cavity by apick up'loop"'28' anda coaxial line l9.

In a"circuit of the type 'shown in Figure" 4 a Western -Electric 707 Atube "was" employedbvith The-end 1"- walls and 21 comprise reflectorsand may be The distances volts. The vacuum tube is mounted in theresonant cavity such as 26 having a rectangular cross-section" sothatthe dimensions A and B were inch and 1%; inches respectively. Thedistances L:"I and: L"2 were so adjusted as to give "a strong output ata frequency of 8600 megacycles. The fundamental frequency of theresonantcavity 2Bwas estimated as being 2150 megacycles. It was observedthat ten per cent variationsin the dimensions L-l and L2 produced nonoticeable efiect in the output frequency thus illustrating that theoutput obtained was ac:- tu'ally a harmonic 'of thefundamentaloscillation frequency. It was also observed that" "in thisparticular case the reflector wall 2| could be omitted withoutdetrimental elfect as faras power output is concerned. The advantage ofthe present invention is-clearly illustrated by' the fact that a tube ofthis type could bemade to operate to supply appreciable power 'attheabove mentioned frequency.

While for the purpose of illustrating, and'describing the presentinvention, certain types of velocity "modulation vacuum-tubes have been;d1- agrarnrnatically indicated and others have" been mentioned inthedescription, and one formf'of resonant cavity has been shown in thedrawing. it is'tobe understood that'the inventionisnbt to be limitedthereby 'sinceobviously the invention is susceptible of such otherembodiments. as to tubes, cavities, and circuits as may be commensuratewith the spirit and scopeiof the in vent'ion set forth in the appendedclaims y This invention is hereby claimed as follows:

1. A. microwave generator, comprising ave locity'rnodulation tube, arectangular cavityresonator extending'transver'selyof said tube andadapted to receive microwave energy therefrom, the cross-sectionaldimensions of said re'so'n'a tor providingresonance at a certainfundam'errtal frequency, the length of said resonator also providingresonance ata harmonic of said fundamental frequency, and a wave guidecoupled to one end of said'res'onator and having thesaniecross-sectional dimensions as said resonator.-'

2. Amicrowave generator, comprising "a'velocity modulation tube, a"rectangular cavity resonator extending transversely of said a tube andadapted to receive microwave energy-there from,'-the cross-sectionaldimensions ofsaid r onator providing resonance at a" certain fundamental frequency, the length of said resonator alsopro'vidi'ng resonanceat a harmonicof said fundamental frequency, and a waveguide cou pled toone end of saidreso'nator- -throng" -arr'iris which raisesthe Q of theresonator at theharmonidfrequen'cy, said wave guide having the samecross-sectional dimensions as said resonator.

3; A *mi'crowanre "generator; comprising a velocity modulation tube, arectangular cav-iryres onator extending-' transversely of said tube "andadaptedto receive microwave energy therefrom. said resonator havinginwardly extending pt'ir tibns for "engagement with neuroses; of f saidtube, the cross-sectional dimensions of saidres onator and the spacingof said portions provlding resonance at a certain fundamental Irequency,the length of said resonator also providlng resonance at a harmonic ofsaid fundamental frequency, and a 'wave guide coupled to one end of saidresonator and having the same cross-sectional dimensions as saidresonator.

4. A microwave generator, comprisin a velocity modulation tube, a cavityresonator extending transversely of the tube and adapted to receivemicrowave energy therefrom, said resonator being dimensionedcross-sectionally to provide resonance at a certain fundamental Ire-,quency, and being dimensioned lengthwise to provide simultaneousresonance at a harmonic of said fundamental frequency, and means coupledto said resonator for deriving energy therefrom at said harmonicfrequency.

DAVID E. SUNSTEIN.

6 REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

